Gypsum-containing products containing alpha hemihydrate

ABSTRACT

The invention provides gypsum wallboard, made from gypsum-containing slurries having outstanding hydration rate characteristics comprising alpha-hemihydrate ground to a particular particle size distribution range and having a Blaine surface area in the range from about 3100 cm 2 /g to about 9000 cm 2 /g, alone or in combination with beta-hemihydrate.

FIELD OF THE INVENTION

This invention pertains to the use of alpha-hemihydrate in makinggypsum-based products. This invention also pertains to a method ofreducing water requirements in slurries used to make gypsum-containingproducts and more particularly to gypsum slurries containingalpha-hemihydrate, alone or in combination with beta-hemihydrate, tomake gypsum wallboard. It also pertains to a method of increasing drystrength of gypsum wallboard using alpha-hemihydrate.

BACKGROUND OF THE INVENTION

Certain properties of gypsum (calcium sulfate dihydrate) make it verypopular for use in making industrial and building products, particularlygypsum wallboard. Gypsum is a plentiful and generally inexpensive rawmaterial which, through a process of dehydration (or calcination) andrehydration, can be cast, molded or otherwise formed into useful shapes.The base material from which gypsum wallboard and other gypsum productsare manufactured is the hemihydrate form of calcium sulfate(CaSO₄.½H₂O), commonly termed “stucco,” which is produced by heatconversion of the dihydrate form of calcium sulfate (CaSO₄.2H₂O), inwhich 1½ water molecules been removed. After rehydration, thehemihydrate dissolves, gypsum crystals precipitate, and the crystal masssets and becomes solid, providing a set gypsum material.

There are two categories of gypsum, alpha-hemihydrate andbeta-hemihydrate, which are produced by different calcinations methods.Alpha-hemihydrate (or alpha gypsum) is calcined under pressure.Beta-hemihydrate (or beta gypsum) is made by calcining in a kettle atatmospheric pressure. The stucco used in making gypsum wallboard isexclusively in the beta-hemihydrate form. Alpha-hemihydrate is not usedcommercially in the production of gypsum wallboard primarily due to itsslower hydration rate compared to beta-hemihydrate (which would requireslower line speed), and the lower strength characteristics obtained whencommonly available alpha-hemihydrate is used at densities common towallboard manufacturing. However, it would be advantageous to be able touse alpha-hemihydrate or blends of alpha-hemihydrate andbeta-hemihydrate in making gypsum wallboard, since alpha-hemihydrate isa readily available raw material having several unique usefulproperties. These unique useful properties include substantially lowerwater demand to produce required fluidity than beta-hemihydrate andresultant set cast higher density, higher strength, and higher surfacehardness.

It is necessary to use substantial amounts of water in gypsum slurriesin order to ensure proper flowability of the slurry. Unfortunately, mostof this water must eventually be driven off by heating, which isexpensive due to the high cost of the fuels used in the heating process.The heating step is also time-consuming. This means that ifalpha-hemihydrate could be used in making wallboard it wouldsubstantially reduce the water demand and hence the expense and timerequired to produce the wallboard.

It now has been found that when alpha-hemihydrate is ground to produceparticles of alpha-hemihydrate as described below, its hydration ratecan be substantially improved without losing any of its other desirableproperties, including its low water demand. Indeed, it has been foundthat alpha-hemihydrate hydration rates can be achieved in slurries usedto make wallboard that are fully acceptable for use in production ofgypsum wallboard.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the invention constitutes gypsum wallboard comprisinga set gypsum composition between two substantially parallel coversheets, the set gypsum composition made using a gypsum-containing slurryof water, and ground alpha-hemihydrate. The alpha-hemihydrate has aparticle size distribution within the following range:

d(0.1)=about 3μ-5μ,

d(0.5)=about 14μ-50μ,

d (0.9)=about 40μ-100μ,

and a Blaine surface area in the range from about 3100 cm²/g to about9000 cm²/g. Other conventional ingredients will also be used in theslurry including, as appropriate, dispersants (such asnaphthalenesulfonates), strength additives (such as trimetaphosphates),accelerators, binders, starch, paper fiber, glass fiber, and other knowningredients. A soap foam can be added to reduce the density of the finalgypsum wallboard product.

In another embodiment the invention constitutes a method of makinggypsum wallboard by mixing a gypsum-containing slurry comprising water,and ground alpha-hemihydrate having a particle size distribution withinthe following range:

d (0.1)=about 3μ-5μ,

d (0.5)=about 14μ-50μ,

d (0.9)=about 40μ-100μ,

and a Blaine surface area in the range from about 3100 cm²/g to about9000 cm²/g. The resulting gypsum-containing slurry is deposited on afirst paper cover sheet, and a second paper cover sheet is placed overthe deposited slurry to form a gypsum wallboard. The gypsum wallboard iscut after the gypsum-containing slurry has hardened sufficiently forcutting, and the resulting gypsum wallboard is dried. Other conventionalingredients will also be used in the slurry including, as appropriate,dispersants (such as naphthalenesulfonates), strength additives (such astrimetaphosphates), accelerators, binders, starch, paper fiber, glassfiber, and other known ingredients. A soap foam can be added to reducethe density of the final gypsum wallboard product.

In yet another embodiment the invention comprises gypsum wallboard andslurries used in making wallboard in which some or all of the stuccocomponent is ground alpha-hemihydrate in the particle size rangesdiscussed above. Where the stucco component is not all alpha-hemihydratethe other stucco component will be beta-hemihydrate. The water demand insuch slurries for making gypsum wallboard is further reduced byintroducing into the slurry about 0.12-0.4% by weight based on theweight of dry stucco of a trimetaphosphate salt along with anaphthalenesulfonate dispersant in an amount of about 0.5-2.5% by weightbased on the weight of dry stucco in the formulation. Other conventionalingredients will also be used in the slurry including, as appropriate,accelerators, binders, starch, paper fiber, glass fiber, and other knowningredients. A soap foam can be added to reduce the density of the finalgypsum wallboard product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting the particle size distribution ofalpha-hemihydrate and beta-hemihydrate samples in one embodiment of theinvention.

FIG. 2 is a graph depicting the hydration rate of a 50:50 blend of fineground alpha-hemihydrate and beta-hemihydrate and the hydration rate of100% alpha-hemihydrate.

FIG. 3 is a graph depicting compressive strength of a 50:50 (w/w) blendof fine ground alpha-hemihydrate and beta-hemihydrate and compressivestrength of 100% beta-hemihydrate.

FIG. 4 is a bar graph depicting slump size as a measure of the fluidityof a gypsum-containing slurry formulation (Formulation B) in oneembodiment of the present invention.

FIG. 5 is a graph depicting compressive strength of gypsum-containingslurry formulation (Formulation A) in one embodiment of the presentinvention.

FIG. 6 is a graph depicting nail pull test data of boards made with a50:50 (w/w) blend of fine ground alpha-hemihydrate and beta-hemihydratein one embodiment of the present invention.

FIG. 7 is a graph depicting nail pull test data of wallboards made with100% ground alpha-hemihydrate in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In the present inventions it unexpectedly has been found that gypsumwallboard can be obtained using alpha-hemihydrate ground to specificparticle size ranges. Any suitable standard commercial grindingequipment can be used for this purpose. Grinding can be achieved byusing a mechanical milling means, for example, such as an impact mill ora ball mill.

Particle size distribution (“PSD”) of the alpha-hemihydrate is acritical feature of the invention and should lie within the followingrange:

d (0.1)=about 3μ-5μ

d (0.5)=about 14μ-50μ

d (0.9)=about 40μ-100μ.

The particle size range can be determined on a Malvern Instruments ModelMastercizer 2000 or other commercially available measuring instrument.

The above values represent percentages by volume, that is: d(0.1)indicates that 10% of the total volume of particles have a diameter lessthan or equal to about 3μ-5μ, while the remaining 90% has a diametergreater than 3μ-5μ; d(0.5) indicates that 50% of the total volume ofparticles have a diameter less than or equal to about 14μ-50μ, while 50%has a diameter greater than 14μ-50μ; and d(0.9) indicates that 90% ofthe total volume of particles have a diameter less than or equal toabout 40μ-100μ, while the remaining 10% has a diameter greater than40μ-100μ.

Preferably, the PSD will lie within the following range:

d (0.1)=about 3μ-5μ

d (0.5)=about 14μ-20μ

d (0.9)=about 40μ-50μ.

One preferred alpha-hemihydrate has a PSD of: d (0.1)=5μ, d (0.5)=50μ, d(0.9)=100μ. Another more preferred alpha-hemihydrate has a PSD of: d(0.1)=5μ, d (0.5)=20μ, d (0.9)=50μ. A yet more preferredalpha-hemihydrate has a PSD of: d (0.1)=3μ, d (0.5)=14μ, d (0.9)=40μ. Aparticularly preferred alpha-hemihydrate has a PSD of: d (0.1)=3μ, d(0.5)=14.1μ, d (0.9)=45.9μ.

Also, the Blaine surface area of the ground particles should beconcomitantly about 3100-9000 cm²/g, preferably about 3500-6000 cm²/gand most preferably about 3900 cm²/g. The Blaine surface area can bedetermined on an instrument available from Humboldt Manufacturing Co.,Norridge, Ill., or other commercially available measuring instrument.

Based on the same PSD analysis, the PSD of commercial gradebeta-hemihydrate is: d (0.1)=2.1μ, d (0.5)=9.2μ, d (0.9)=49.1μ.Commercial grade ground alpha-hemihydrate typically has the followingPSD: d (0.1)=4.4μ, d (0.5)=36.8μ, d (0.9)=169μ, while ungroundalpha-hemihydrate has the following PSD: d (0.1)=17.4μ, d (0.5)=64.5μ, d(0.9)=162.8μ. Commercial grade ground alpha-hemihydrate typically has aBlaine surface area of about 2700 cm²/g. These values all fall outsideof the useful range in the embodiments of the present invention.

In embodiments where alpha-hemihydrate of the invention is used incombination with beta-hemihydrate, the alpha-hemihydrate andbeta-hemihydrate preferably will be blended before introduction into theslurry. Any suitable standard commercial blending equipment, or similarapparatus, can be used for this purpose. For experimental purposes, forexample, fine ground alpha-hemihydrate and beta-hemihydrate can be addedto a plastic bag, which is then sealed and shaken manually to preparethe blend. A particularly preferred blend of alpha-hemihydrate andbeta-hemihydrate is 50:50 (w/w).

Water/stucco (w/s) ratio, or “WSR,” is an important economic parameter,since excess water must eventually be driven off by heating, which isexpensive due to the high cost of the fuels used in the heating process.It is advantageous for the amount of process water, and consequentlyWSR, to be kept low. In the embodiments of the present invention, WSRcan range from about 0.2 to about 1.0. In a preferred embodiment, WSRcan range from about 0.4 to about 0.5, which range demonstrates asubstantially lower water demand. In addition, it has been found thatgypsum slurries made using alpha-hemihydrate in accordance with thepresent invention maintain excellent fluidity at very low WSR, fromabout 0.2 to about 0.3, for example. Gypsum wallboard made using theslurries also exhibit excellent compressive strength.

The combination of a minimum of at least about 0.12-0.4% by weight oftrimetaphosphate salt and from about 0.5%-2.5% by weightnaphthalenesulfonate dispersant (both based on the weight of dry stuccoused in the gypsum slurry) unexpectedly and significantly increases thefluidity of the gypsum slurry beyond the already significant improvementin fluidity achieved using alpha-hemihydrate in accordance with thepresent invention. This further reduces the amount of water required toproduce a gypsum slurry with sufficient flowability to be used in makinggypsum wallboard. The level of trimetaphosphate salt, which is at leastabout twice that of standard formulations (as sodium trimetaphosphate),is believed to boost the dispersant activity of the naphthalenesulfonatedispersant. It should be noted that in all embodiments of the presentinvention, a combination of both naphthalenesulfonate dispersant andwater-soluble metaphosphate or polyphosphate (and preferably awater-soluble trimetaphosphate) must be used.

The naphthalenesulfonate dispersants used in the present inventioninclude polynaphthalenesulfonic acid and its salts(polynaphthalenesulfonates) and derivatives, which are condensationproducts of naphthalenesulfonic acids and formaldehyde. Particularlydesirable polynaphthalenesulfonates include sodium and calciumnaphthalenesulfonate. The average molecular weight of thenaphthalenesulfonates can range from about 3,000 to 20,000, although itis preferred that the molecular weight be about 8,000 to 10,000. Ahigher molecular weight dispersant has higher viscosity, and generates ahigher water demand in the formulation. Useful naphthalenesulfonatesinclude LOMAR D, available from Henkel Corporation, DILOFLO, availablefrom GEO Specialty Chemicals, Cleveland, Ohio, and DAXAD, available fromHampshire Chemical Corp., Lexington, Mass. It is preferred that thenaphthalenesulfonates be used in the form of an aqueous solution, forexample, in the range of about 40-45% by weight solids content.

The polynaphthalenesulfonates useful in the present invention have thegeneral structure (I):

wherein n is >2, and wherein M is sodium, potassium, calcium, and thelike.

The naphthalenesulfonate dispersant must be used in a range of fromabout 0.5% to about 2.5% by weight based on the weight of dry stuccoused in the gypsum composite formulation. A preferred range ofnaphthalenesulfonate dispersant is from about 0.5% to about 1.5% byweight based on the weight of dry stucco, a more preferred range fromabout 0.7% to about 1.5% by weight based on the weight of dry stucco,and a most preferred range from about 0.7% to about 1.2% by weight basedon the weight of dry stucco.

Any suitable water-soluble metaphosphate or polyphosphate can be used inaccordance with the present invention. It is preferred that atrimetaphosphate salt be used, including double salts, that istrimetaphosphate salts having two cations. Particularly usefultrimetaphosphate salts include sodium trimetaphosphate, potassiumtrimetaphosphate, calcium trimetaphosphate, sodium calciumtrimetaphosphate, lithium trimetaphosphate, ammonium trimetaphosphate,and the like, or combinations thereof. A preferred trimetaphosphate saltis sodium trimetaphosphate. It is preferred to use the trimetaphosphatesalt as an aqueous solution, for example, in the range of about 10-15%by weight solids content. Other cyclic or acyclic polyphosphates canalso be used, as described in U.S. Pat. No. 6,409,825 to Yu et al.,herein incorporated by reference.

Sodium trimetaphosphate is a known additive in gypsum-containingcompositions, although it is generally used in a range of from about0.05% to about 0.08% by weight based on the weight of dry stucco used inthe gypsum slurry. In embodiments of the present invention, sodiumtrimetaphosphate (or other water-soluble metaphosphate or polyphosphate)must be present in the range of from about 0.12% to about 0.4% by weightbased on the weight of dry stucco used in the gypsum compositeformulation. A preferred range of sodium trimetaphosphate (or otherwater-soluble metaphosphate or polyphosphate) is from about 0.12% toabout 0.3% by weight based on the weight of dry stucco used in thegypsum composite formulation.

Starches, including pregelatinized starch in particular, can be used ingypsum-containing slurries prepared in accordance with the presentinvention. A preferred pregelatinized starch is pregelatinized cornstarch, for example pregelatinized corn flour available from Bunge, St.Louis, Mo., having the following typical analysis: moisture 7.5%,protein 8.0%, oil 0.5%, crude fiber 0.5%, ash 0.3%; having a greenstrength of 0.48 psi; and having a loose bulk density of 35.0 lb/ft³.Pregelatinized corn starch can be used in an amount up to about 10% byweight, based on the weight of dry stucco used in the gypsum-containingslurry.

Other useful starches include acid-modified starches, such asacid-modified corn flour, available as HI-BOND from Bunge, St. Louis,Mo. This starch has the following typical analysis: moisture 10.0%, oil1.4%, solubles 17.0%, alkaline fluidity 98.0%, loose bulk density 30lb/ft³, and a 20% slurry producing a pH of 4.3. Another useful starch isnon-pregelatinized wheat starch, such as ECOSOL-45, available fromADM/Ogilvie, Montreal, Quebec, Canada, having maximum solubles 25.0%.

A further unexpected result may be achieved with the present inventionwhen the naphthalenesulfonate dispersant trimetaphosphate saltcombination is combined with pregelatinized corn starch, and optionally,paper fiber or glass fiber. Gypsum wallboard made from formulationscontaining these three ingredients have increased strength and reducedweight, and are more economically desirable due to the reduced waterrequirements in their manufacture.

Accelerators can be used in the gypsum-containing compositions of thepresent invention, for example, wet gypsum accelerator (WGA), asdescribed in U.S. Pat. No. 6,409,825 to Yu et al., herein incorporatedby reference. One desirable heat resistant accelerator (HRA) can be madefrom the dry grinding of landplaster (calcium sulfate dihydrate). Smallamounts of additives (normally about 5% by weight) such as sugar,dextrose, boric acid, and starch can be used to make this HRA. Sugar ordextrose are currently preferred. Another useful accelerator is “climatestabilized accelerator” or “climate stable accelerator,” (CSA) asdescribed in U.S. Pat. No. 3,573,947, herein incorporated by reference.

Gypsum wallboard made according to the embodiments of the presentinvention includes cover sheets or surface sheets, between which a setgypsum core is formed from a gypsum-containing slurry. In accordancewith the invention, the gypsum-containing slurries will include groundalpha-hemihydrate with particles sizes as described above, or blends ofsuch alpha-hemihydrate and beta-hemihydrate. The set gypsum-containingcore material is sandwiched between two substantially parallel coversheets, for example paper cover sheets. Various types of paper coversheets are known in the art and all such types of paper cover sheets maybe used in the present invention. Cover sheets comprising mats of glassor polymer fibers may also be used.

The following examples further illustrate the invention. They should notbe construed as in any way limiting the scope of the invention.

Calcination technology provides an economical way of producingalpha-hemihydrate. However, commercial grade, plant producedalpha-hemihydrate cannot be easily hydrated in a manner that is requiredin wallboard production. It has been found that grinding the regularalpha-hemihydrate, as shown in Example 1, to a desired particle sizedistribution (“PSD”), as shown in FIG. 1, speeds up the hydrationprocess, as shown in FIG. 2 and Table 1 below.

EXAMPLE 1

Preparation of Fine Ground Alpha-Hemihydrate

Unground alpha-hemihydrate is ground with a Vortec M-1 impact mill,available from Vortec Industries of Long Beach, Calif., at 60 Hz speedsetting @ 1.8 lb./min. The PSD of raw material and fine ground materialis shown in FIG. 1. The resulting fine ground alpha-hemihydrate isblended with beta-hemihydrate at a 50:50 (wt/wt) ratio by labscale twinshell mixer.

As shown in FIG. 1, the PSD of fine ground alpha-hemihydrate closelyresembles the PSD of beta-hemihydrate. Unground alpha-hemihydrate isalso shown for comparison.

As shown in FIG. 2, the hydration rate of a 50:50 blend of fine groundalpha-hemihydrate and beta-hemihydrate is reduced substantially from100% alpha-hemihydrate, even though the 100% alpha-hemihydrate samplehas also been fine ground. Hydration rate was determined according tothe test procedure found in Example 2 of U.S. Pat. No. 6,815,049 toVeeramasuneni et al., herein incorporated by reference.

Table 1 demonstrates the improvement in hydration time for exemplaryblends. TABLE 1 Time to 50% Time to 98% Hydration, min. Hydration, min.Alpha component of 50:50 Stucco sample Regular unground alpha-2.5-3.5 >10 hemihydrate w/0.5% CSA and 0.5% Potash (or sodium bisulfate)33 Hz Vortec ground 3.75 11.67 alpha w/0.5% CSA 60 Hz Vortec fine ground4 9.67 alpha w/0.5% CSA 60 Hz Vortec fine ground 3.25 8.83 alpha w/0.5%CSA and 0.5% Potash 60 Hz Vortec fine ground 3.50 7.75 alpha w/1%landplaster and 0.5% Potash Conventional Wallboard Slurry No alpha (onlyBeta 3-4 5-6.5 stucco w/WGA)All 50:50 blends included 1.0% LOMAR D by weight.

As shown in Table 1, the time to 98% hydration (kiln) was reduced fromabout 12 min. to 8.8 minutes as blends were optimized. In fact, when thefine ground alpha-hemihydrate included uncalcined gypsum (CaSO₄.2H₂O,i.e. “landplaster”), the hydration was even faster at 7.8 min.Therefore, fine grinding of the alpha-hemihydrate solves the issue ofslow hydration rate.

EXAMPLE 2

Compressive Strength of Alpha/Beta Blends

With reference to FIG. 3, a 50:50 (w/w) blend of fine groundalpha-hemihydrate and beta-hemihydrate is shown to be generallyequivalent in bench cube strength to 100% beta-hemihydrate. Compressivestrength in psi, as reported in FIG. 1, was determined using neat stuccocubes made with water and stucco only (no foam) at varying densities inpounds per cubic foot (pcf). A 50:50 (w/w) blend of ungroundalpha-hemihydrate and beta-hemihydrate yielded poor strength results.

EXAMPLE 3

Sample Gypsum Slurry Formulation

Gypsum slurry formulations are shown in Table 2 below. All values inTable 2 are expressed as weight percent based on the total weight of drystucco. TABLE 2 Formulation A, Formulation B, Component wt. % wt. %50:50 (w/w) blend of 100 100 fine ground alpha- hemihydrate and beta-hemihydrate sodium 0.30 0.30 trimetaphosphate Dispersant 0.5 2.5(naphthalenesulfonate) Pregelatinized starch 1.0 3.0 climate stable 0.50.5 accelerator (CSA) Water/Stucco ratio 0.5 0.5

EXAMPLE 4

Effect of Formulation B on Water Demand

As shown in Table 2, high levels of trimetaphosphate salt and starch, asin Formulation B, were used to prepare gypsum-containing slurries.Slurry compositions such as Formulation B were found to have excellentfluidity at low WSR. As shown in FIG. 4, water demand was heldsubstantially low using, for example, Formulation B. In order to measurefluidity in the slurry, a slump test was performed as follows.

Slump test. This test was performed using gypsum board core slurry atthe mixer. The test was performed on a 12×12 inch Plexiglass plate sothat slurry diameter can be measured without waiting for the slurry toset. Slurry was drawn from as close to the mixer as possible. A 2 inchby 4 inch smooth-walled brass or PVC cylinder mold was quickly filledwith test slurry sample, and any overflow leveled off. The cylinder moldwas then lifted rapidly straight up to give a gypsum patty. The diameterof the gypsum patty was measured. The resultant gypsum patty will be inthe range of about 5 to 10 inches in diameter. This test was repeateduntil the results of three consecutive tests were within ⅛ inch, andthis value was then recorded as slump diameter (slump size). The entiretest procedure should not take more than 15 seconds to perform.

EXAMPLE 5

Effect of Formulation A on Compressive Strength

Slurry compositions such as Formulation A (Table 2) were found toexhibit superior compressive strengths when used in cube tests. As shownin FIG. 5, compressive strengths at varied cube densities were at leastabout 10% greater when Formulation A was used, in comparison to testshaving no starch or no starch and no dispersant. It should be stressedthat naphthalenesulfonate dispersant is always needed in order toachieve a low WSR in the slurry.

Compressive strength was determined according to ASTM C-472, and inaccordance with U.S. Pat. No. 6,815,049 to Veeramasuneni et al., hereinincorporated by reference.

EXAMPLE 6

Nail Pull Tests of Wallboard Prepared with 50:50 Blend Fine GroundAlpha-Hemihydrate and Beta-Hemihydrate

Sample gypsum wallboards were prepared in accordance with U.S. Pat. No.6,342,284 to Yu et al. and U.S. Pat. No. 6,632,550 to Yu et al., hereinincorporated by reference. This includes the separate generation of foamand introduction of the foam into the slurry of the other ingredients asdescribed in Example 5 of these patents.

In order to demonstrate superior performance using the 50:50 (w/w) blendof fine ground alpha-hemihydrate and beta-hemihydrate, anaphthalenesulfonate dispersant, and a trimetaphosphate salt, boardsamples were prepared at 0.472 WSR. As shown in FIG. 6, boards made withthe 50:50 (w/w) blend of fine ground alpha-hemihydrate andbeta-hemihydrate, 1% by weight naphthalenesulfonate dispersant based onthe weight of stucco, and 0.3% by weight trimetaphosphate salt based onthe weight of stucco provided much better nail pull values than boardsprepared using 50:50 (w/w) blends of regular (plant ground)alpha-hemihydrate and beta-hemihydrate (and the same additives) at 0.55WSR. Two sets of test boards were prepared using the fine groundalpha-hemihydrate blend with beta-hemihydrate.

Nail pull resistance tests were performed according to ASTM C-473.Additionally, it is noted that typical gypsum wallboard is approximately½ inch thick and has a weight of between about 1600 to 1800 pounds per1,000 square feet of material, or lb/MSF. (“MSF” is a standardabbreviation in the art for a thousand square feet; it is an areameasurement for boxes, corrugated media and wallboard.)

EXAMPLE 7

Slurries Prepared Using 100% Fine Ground Alpha-Hemihydrate

If 100% fine ground alpha-hemihydrate, as prepared in Example 1, wereused in slurry formulations, it is anticipated that a much lower waterdemand would result compared to slurries made using beta-hemihydrate. Inaddition, if 100% fine ground alpha-hemihydrate were used in slurryformulations including a trimetaphosphate salt and anaphthalenesulfonate dispersant, such as in Example 3 above, it isanticipated that water demand would decrease even further, that is, WSRin the range of about 0.2 to about 0.3. As shown in FIG. 7, wallboardsmade with 100% ground alpha-hemihydrate in accordance with the presentinvention provided excellent nail pull values that meet or exceedindustry standards. Three sets of test boards were prepared using 100%ground alpha-hemihydrate.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. Recitation of ranges of values herein are merely intended toserve as a shorthand method of referring individually to each separatevalue falling within the range, unless otherwise indicated herein, andeach separate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context. The use of any and allexamples, or exemplary language (e.g., “such as”) provided herein, isintended merely to better illuminate the invention and does not pose alimitation on the scope of the invention unless otherwise claimed. Nolanguage in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention. Itshould be understood that the illustrated embodiments are exemplaryonly, and should not be taken as limiting the scope of the invention.

1. A gypsum wallboard comprising: a set gypsum composition formedbetween two substantially parallel cover sheets, the set gypsumcomposition made using a gypsum-containing slurry comprising: water, andground alpha-hemihydrate having a particle size distribution within thefollowing range: d (0.1)=about 3μ-5μ, d(0.5)=about 14μ-50μ, d(0.9)=about 40μ-100μ, and a Blaine surface area in the range from about3100 cm²/g to about 9000 cm²/g.
 2. The gypsum wallboard of claim 1,wherein the ground alpha-hemihydrate has a particle size distributionwithin the following range: d (0.1)=about 3μ-5μ, d(0.5)=about 14μ-20μ, d(0.9)=about 40μ-50μ, and a Blaine surface area in the range from about3500 cm²/g to about 6000 cm²/g.
 3. The gypsum wallboard of claim 1,wherein the ground alpha-hemihydrate has a particle size distribution ofd (0.1)=about 3μ, d (0.5)=about 14.1μ, d (0.9)=about 45.9μ, and a Blainesurface area of about 3900 cm²/g.
 4. The gypsum wallboard of claim 1,wherein the gypsum-containing slurry further comprises sodiumtrimetaphosphate present in an amount of at least about 0.12% by weightbased on the weight of alpha-hemihydrate, and a naphthalenesulfonatedispersant present in an amount from about 0.5% to about 2.5% by weightbased on the weight of alpha-hemihydrate.
 5. The gypsum wallboard ofclaim 4, wherein the gypsum-containing slurry further includes a starch.6. The gypsum wallboard of claim 5, wherein the starch is pregelatinizedcorn starch present in an amount up to about 6% by weight based on theweight of alpha-hemihydrate.
 7. The gypsum wallboard of claim 1, whereinthe gypsum-containing slurry further comprises beta-hemihydrate.
 8. Thegypsum wallboard of claim 7, wherein the ratio of alpha-hemihydrate tobeta-hemihydrate is about 50:50 (w/w).
 9. The gypsum wallboard of claim7, wherein the gypsum-containing slurry further comprises sodiumtrimetaphosphate present in an amount of at least about 0.12% by weightbased on the total weight of alpha-hemihydrate and beta-hemihydrate, anda naphthalenesulfonate dispersant present in an amount from about 0.5%to about 2.5% by weight based on the total weight of alpha-hemihydrateand beta-hemihydrate.
 10. The gypsum wallboard of claim 8, wherein thegypsum-containing slurry further comprises sodium trimetaphosphatepresent in an amount of at least about 0.12% by weight based on thetotal weight of alpha-hemihydrate and beta-hemihydrate, and anaphthalenesulfonate dispersant present in an amount from about 0.5% toabout 2.5% by weight based on the total weight of alpha-hemihydrate andbeta-hemihydrate.
 11. The gypsum wallboard of claim 10, wherein thegypsum-containing slurry further includes a starch.
 12. The gypsumwallboard of claim 11, wherein the starch is pregelatinized corn starchpresent in an amount up to about 6% by weight based on the total weightof alpha-hemihydrate and beta-hemihydrate.
 13. The gypsum wallboard ofclaim 1, wherein the cover sheets comprise paper.
 14. Agypsum-containing slurry comprising: water, and ground alpha-hemihydratehaving a particle size distribution within the following range: d(0.1)=about 3μ-5μ, d (0.5)=about 14μ-50μ, d (0.9)=about 40μ-100μ, and aBlaine surface area in the range from about 3100 cm²/g to about 9000cm²/g.
 15. The gypsum-containing slurry of claim 14, wherein the groundalpha-hemihydrate has a particle size distribution within the followingrange: d (0.1)=about 3μ-5μ, d(0.5)=about 14μ-20μ, d (0.9)=about 40μ-50μ,and a Blaine surface area in the range from about 3500 cm²/g to about6000 cm²/g.
 16. The gypsum-containing slurry of claim 14, wherein theground alpha-hemihydrate has a particle size distribution of d(0.1)=about 3μ, d (0.5)=about 14.1μ, d (0.9)=about 45.9μ, and a Blainesurface area of about 3900 cm²/g.
 17. The gypsum-containing slurry ofclaim 14, further comprising sodium trimetaphosphate present in anamount of at least about 0.12% by weight based on the weight ofalpha-hemihydrate, and a naphthalenesulfonate dispersant present in anamount from about 0.5% to about 2.5% by weight based on the weight ofalpha-hemihydrate.
 18. The gypsum-containing slurry of claim 14, furthercomprising beta-hemihydrate.
 19. A method of making gypsum wallboard,comprising the steps of: (a) mixing a gypsum-containing slurrycomprising water, and ground alpha-hemihydrate having a particle sizedistribution within the following range: d (0.1)=about 3μ-5μ, d(0.5)=about 14μ-50μ, d (0.9)=about 40μ-100μ, and a Blaine surface areain the range from about 3100 cm²/g to about 9000 cm²/g; (b) depositingthe gypsum-containing slurry on a first cover sheet; (c) placing asecond cover sheet over the deposited slurry to form a gypsum wallboard;(d) cutting the gypsum wallboard after the gypsum-containing slurry hashardened sufficiently for cutting; and (e) drying the gypsum wallboard.20. The method of claim 19, wherein the ground alpha-hemihydrate has aparticle size distribution within the following range: d(0.1)=about3μ-5μ, d (0.5)=about 14μ-20μ, d (0.9)=about 40μ-50μ, and a Blainesurface area in the range from about 3500 cm²/g to about 6000 cm²/g. 21.The method of claim 19, wherein the ground alpha-hemihydrate has aparticle size distribution of d (0.1)=about 3μ, d (0.5)=about 14.1μ, d(0.9)=about 45.9μ, and a Blaine surface area of about 3900 cm²/g. 22.The method of claim 19, wherein the gypsum-containing slurry furthercomprises sodium trimetaphosphate present in an amount of at least about0.12% by weight based on the weight of alpha-hemihydrate, and anaphthalenesulfonate dispersant present in an amount from about 0.5% toabout 2.5% by weight based on the weight of alpha-hemihydrate.
 23. Themethod of claim 19, wherein the gypsum-containing slurry furthercomprises beta-hemihydrate.
 24. The method of claim 19, in which thefirst cover sheet and the second cover sheet are made of paper.